The present invention relates to methods and apparatuses for detecting topographical features of microelectronic substrates, for example, detecting the surface roughness of a microelectronic substrate having solder or gold bump terminals. Packaged microelectronic assemblies, such as memory chips and microprocessor chips, typically include a microelectronic substrate die encased in a plastic, ceramic, or metal protective covering. The dies are typically formed in or on a wafer, such as a silicon wafer, and can include functional devices or features, such as memory cells, processing circuits, and interconnecting wiring. Each die also typically includes bond pads or other conductive structures, such as gold bumps or solder bumps that are electrically coupled to the functional devices. The conductive structures can then be electrically coupled to pins or other types of terminals that extend outside the protective covering for connecting to buses, circuits, and/or other microelectronic assemblies.
One method for increasing the throughput of packaged microelectronic assemblies is to perform many processing operations on the dies before the dies are singulated from the wafer, a practice referred to in the industry as wafer-level packaging. One such process step includes disposing gold or solder bumps on the dies at the wafer level to form a “bumped” wafer. When performing such operations at the wafer level, it is typically important to measure the average thickness, thickness variation, and roughness of the wafer to ensure that the wafer meets tight dimensional specifications, and to ensure that any microdefects of the wafer (which can reduce wafer strength) are eliminated or reduced to acceptable levels.
FIG. 1A is a schematic illustration of a conventional apparatus 10a for measuring the thickness and thickness variation of a wafer 30. Such apparatuses are available from ADE of Westwood, Mass., under model numbers 9520 and 9530. The apparatus 10a can include a narrow, rod-shaped vacuum chuck 12 that supports the wafer 30, a lower capacitance probe 11a that measures the distance to the wafer back surface, and an upper capacitance probe 11b that measures the distance to the wafer front or device-side surface. The thickness of the wafer 30 at a particular point on the wafer can be calculated by subtracting the two distance measurements from the total distance between the capacitance probes 11a and 11b. The total thickness variation (TTV) of the wafer 30 can be calculated by traversing the rotating wafer 30 in between the probes 11a and 11b, determining a maximum thickness value and a minimum thickness value, and subtracting the minimum thickness value from the maximum thickness value. The average thickness of the wafer can be calculated by taking the mean of all the thickness values collected.
FIG. 1B is a schematic illustration of an apparatus 10b used to determine the roughness of the wafer 30. The apparatus 10b can include a support table 20 that carries the wafer 30 with the back surface of the wafer 30 facing upwardly. A stylus 41 traverses over the back surface of the wafer 30 and moves up and down as it passes over roughness features on the back surface. A light 12 illuminates the back surface of the wafer 30 for visual inspection through a microscope 13 which can be used to capture a video image that can be saved on a bitmap file for correlating with the capacitance scan data. Such apparatuses are available from Veeco-Metrology Group of Santa Barbara, Calif.
One drawback with the devices 10a and 10b described above is that they may not be suitable for detecting the characteristics of bumped wafers which have solder bumps or gold bumps that project from a surface of the wafer. For example, the apparatus 10a shown in FIG. 1A typically cannot distinguish between the surface of the wafer 30 and the elevated surface of the bumps on the wafer 30, and can accordingly produce erroneous thickness and thickness variation measurements. The capacitance probes 11a and 11b typically do not have the high resolution required to determine surface roughness. The apparatus 10b shown in FIG. 1B typically includes a vacuum system in the support table 20 to draw the wafer 30 tightly down against the table 20. When the wafer 30 includes solder bumps or gold bumps, the bumped surface of the wafer 30 may not form an adequate seal with the support table 20. Furthermore, the contact between the support table 20 and the wafer 30 can damage the bumps and render all or part of the wafer 30 inoperable.
FIG. 1C illustrates a conventional apparatus 10c available from August Technology of Bloomington, Minn., and specifically configured to detect characteristics of a bumped wafer 30. The apparatus 10c can include a support table 20 having a vacuum system to draw the back surface of the wafer 30 down tightly against the support table 20, with the bumps 34 facing upwardly. A two-dimensional inspection camera 43 traverses above the device-side surface of the wafer 30 to assess the position, diameter, and/or surface characteristics of the bumps 34. A three-dimensional inspection camera 44 can traverse above the device-side surface of the wafer 30 to determine the height of the bumps 34.
One drawback with the device 10c shown in FIG. 1C is that it is not configured to determine the thickness, the total thickness variation, or the roughness of the backside of the wafer 30. Accordingly, none of the apparatuses described above with reference to FIGS. 1A-C are capable of adequately determining the characteristics of the wafer 30 typically used to assess whether the wafer 30 is ready for singulation and subsequent packaging operations.